Ambient Occlusion vs Environment Lighting?
I wouldn't think of Ambient Occlusion as part of your scene lighting, it's really a tool to art direct your image, and (if you output it to a render pass) for extra flexibility in the post production of your image. As Duarte already mentioned it is not physically accurate but is much faster to calculate so it can be helpful to increase the visual appeal of the render.

If you look at many artists' making of videos, you'll generally see that the AO pass is added on in post and not usually as part of the scene render. It's technically a localized shadow effect that gives you more control over the contrast in crevices and surfaces that are close to each other. 

I generally think of it as a tool to give you more control over the image, it's more of an art direction tool than a realistic lighting tool. I think it's more helpful to think of it that way. I know you can have AO enabled and combined into your raw render, but it's much more powerful as a post effect.